Process of treating pigments



Patented Jan. 13, 1942 PROCESS OF TREATING PIGMENTS Kenneth S. Mowlds, Baltimore, Md, assignor to The Glidden Company, Cleveland, Ohio, a corporation of Ohio No Drawing. Application July 27, 1939, Serial No. 286,808

Claims.

The present invention relates to the treatment of pigments, and particularly relates to the treatment of pigments to improve their hiding power.

In copending application Serial No. 175,246, filed November 18, 1938, there is disclosed the treatment of a base pigment with soluble silicates followed by a precipitation of the silicate in insoluble form by means of precipitating salts and acids. Among the salts which may be used is aluminum sulfate. However, certain difiiculties are encountered in the use of aluminum sulfate, and as the pigment produced by using aluminum sulfate has certain advantages over those produced by the use of other precipitating salts it is very desirable that aluminum sulfate be used.

According to the present invention a process is provided whereby theprocess of treating pigment with, soluble silicates followed by precipitation with aluminum sulfate is improved.

In effecting theprecipitation of soluble silicates such as sodiumsilicate, with calcium or other alkaline earth salts, the amount of precipitating salt required for, complete precipitation and coagulation of the sodium silicate is equivalent to the NazO content of the soluble silicate. The soluble silicate does reactas such, but the reaction with precipitating salts appears to be with the Nazi) of the soluble silicate and in the case of calcium and similar salts precipitation is complete When the amount of saltis suflicient to supply enough acid to satisfy the hydroxyl ions provided by the NazO of the soluble silicate. Also the precipitate formed coagulates and is readily filtered.

In efiecting the precipitation of the soluble silicate, such as sodium silicate for example, with an amount of Al2(SO4)3 sufiicient to supply enough acid to satisfy the hydroxyl ions attributable to the NazO in the silicate, if the ratio of Na2O to SiOz is approximately 1:1, the material will precipitate and is fairly .easy to handle on filtration etc. However, it is not as easy to handle as when other precipitation salts such as CaClz, are used. This is believed to be due to the amphoteric nature ofthe A12(SO4)3. As the ratio of NazO to $1102 decreases, the difiiculty of precipitating and handling increases with decreasing NazO content, until at an NazOtSiOz ratio of 1:3 or less precipitation will not occur when using an amount of Al2 SOi 3 just sulficient to supply enough acid to-satisfy the hydroxyl ions attributable to the NazO content.

It has been found, howeventhat if an excess over that amount of A12(SO4)3' isadded "to en fect precipitation, the precipitation is improved. Preferably the amount of A12(SO4)3 added is about 3 times that required to supply suflicient acid to satisfy hydroxyl ions attributable to the NazO content. This amount, however, can be varied and will depend somewhat upon the NazO content of the silicate. The treatment is most needed when the ratio of ,NazO to SiOz is less than 5:7 but advantageous results are obtained by treating silicates in which the ratio is 1:1. By using three times the molecular equivalent of Al2(SO4)3, a sodium silicate having NazO to SiQ-z ratio 1:3 or less can be precipitated.

The difficulty of filtering and handling the productwhen using AI2 (SO4)3 alone toefiect precipitation is present regardless of how this precipitation is brought about. Consequently while precipitation can be effected in the manner indicated and can be employed in the production of a satisfactory pigment of increased hiding power it is desirable that a precipitate be produced which is easier to filter and handle. This difliculty in filtering and handling is greater in those precipitates formed from soluble silicates having lower NazOzsioz ratios than in those having higher ratios.

Since an excess of aluminum sulfatehas been used the precipitate is acid, and it has been found that the difficulty of filtering and handling the precipitated aluminum silicate can be overcome by the addition of a small. amount of an. electrolyte such as alkaline earth metal or zinc oxide, hydroxide or salt, alkali metal oxides and hydroxides such as ZnSO4, BaOH, C'aOH, NaOH, etc. This improvement in the ease of filtering and handling results from coagulation of the precipitate and it will of course be obvious that silicates and insoluble salts such as barium sulfate and calcium carbonate will have little or no coagulating or precipitating action. Inthose instances in which the amount of A12(SO4)3 is sufiicient to substantially completely precipitate the silicate the amount of alkali metal,,alkaline earth metal or zinc compound added may be small and when alkalies are used they are prefera-bly sufficient to supply enough hydroxyl' ions to satisfy the acidity attributable to any excess aluminum sulfate.

In cases where the amount of A12(SO4)3 is not sufiicient to completely precipitate the silicate the amount of the alkaline earth metal or zinc compound should be increased for best results, the amount being increased as the deficiency of Al2(SO4)3 necessary to effect precipitation increases. In such cases the effect of the alkaline cate, particularly where there is an excess of l the A12(SO4)3 over that required to supply enough acid to satisfy the hydroxyl ions attributable to the alkali content of the soluble silicate. This action is also present when alkali metal alkalies are used. Thirdly, the alkaline earth or zinc compound added improves the ease of filtering and handling the precipitate. Thus a simultaneous precipitation of aluminum silicate and the alkaline earth metal or zinc silicate may be employed, although ordinarily a substantially complete precipitation as aluminum silicate is to be preferred. Thus alkalies such as those of alkaline earth metal and zinc which produce insoluble silicates are preferred.

The following examples are illustrative:

Example I 30,000 grams of lithopone were slurried in water and 780 grams of sodium silicate (Naz:SiO2 ratio=1:3.22)

was added. To this slurry was then added 1800 grams of A12(SO4)3.18H2O and 2550 grams of Ba(OH)2.8HzO. After precipitation and coagulation was complete the pigment was filtered, washed and dried. The final pH of the dry pigment was 9.2.

Example I] 3540 grams of lithopone were slurried in water and 92 cc. of sodium silicate solution added. The sodium silicate solution contained solids and the NazO:SiO2 ratio was 123.22. To the slurry was then added 212 grams of A12(SO4)3, 8 H20 and 80 grams of NaOH. After the precipitation was complete the pigment was filtered, washed and dried. The final pH of the dry pigment was 7.7.

The pigment treated may be a white pigment such as lithopone, zinc sulfide, TiOz, or so-called inert pigments, such as calcite, calcium and barium sulfates, asbestine, etc. or mixtures thereof. Treated pigments may be mixed with untreated pigments to produce a composite pigment of improved hiding power. A treated mixture of white and inert pigments has better covering than a mixture of separately treated white pigment and inert pigment, although each mixture has better covering than a mixture of the untreated pigments.

Soluble silicates other than sodium silicate may be used, such as potassium silicate. The amount of soluble silicate may vary, the amounts preferably being such that it contains between about 0.3% and about 10% of SlOz by weight of the pigment. Ordinarily less than 0.3% will not be used since the increase in hiding does not warrant the treatment. Within this range amounts which give smaller SiOz contents in the pigment are preferred, about 0.5% to about 10% SiOz by weight of the pigment being the most desirable. The actual amount used will, of COlll'SG, depend somewhat upon the alkali metal oxide SiOz ratio, and upon the particular alkali metal present,

The soluble silicate is preferably added before the aluminum sulfate is added but the precipitant may be added first and the soluble silicate then added.

Having described the claimed is:

1. The method of improving the hiding power of a pigment which comprises mixing the pigment in an aqueous slurry with an alkali metal silicate in which the ratio of alkali metal oxide to S102 is less than 5:7 and precipitating the silicate by means of aluminum sulfate, the amount of A12(SO4)3 being in excess of that required to supply enough acid to satisfy the hydroxyl ions attributable to the alkali metal oxide of the alkali metal silicate, all in the presence of water.

2. The method of improving the hiding power of a pigment which comprises mixing the pigment in an aqueous slurry with an alkali metal silicate in which the ratio of alkali metal oxide to S102 is less than 5:7 and precipitating the silicate by means of aluminum sulfate, the amount of AMSOQ; being three times that required to supply enough acid to satisfy the hyinvention, what is droxyl ions attributable to the alkali metal oxide in the soluble silicate, all in the presence of water.

3. The method of improving the hiding power of a pigment which comprises mixing the pigment in an aqueous slurry with a water soluble silicate, precipitating the silicate by means of aluminum sulfate, and mixing therewith a small amount of a water soluble substance selected from the class consisting of zinc and alkaline earth metal oxides, hydroxides and salts, and alkali metal oxides and-hydroxides, in amounts sufficient to effect coagulation of the precipitate.

4. The method of improving the hiding power of a pigment which comprises mixing the pigment in an aqueous slurry with an alkali metal silicate in which the ratio of alkali metal oxide to $102 is less than 5 to 7, and precipitating the silicate by means of aluminum sulfate, the amount of aluminum sulfate being in excess of 3 that requiredto supply enough acid to satisfy the hydroxyl ions attributable to the alkali metal oxide of the alkali metal silicate, and adding a small amount of a water soluble substance selected from the class consisting of zinc, and alkaline earth metal oxides, hydroxides and salts, and alkali metal oxides and hydroxides in amounts suificient to effect coagulation of the precipitate.

5. The method of claim 4 in which the amount of A12(SO4)3 is three times that required to supply enough acid to satisfy the hydroxyl ions attributable to the alkali metal oxide of the alkali metal silicate.

6. In the method of improving the hiding power of pigments by treatment of an aqueous slurry of the pigment containing a soluble silicate with aluminum sulfate to effect precipitation of the soluble silicate as aluminum silicate, the improvement which comprises adding a small amount of a water soluble substance selected from the class consisting of zinc and alkaline earth metal oxides, hydroxides and salts and alkaline metal oxides and hydroxides in amounts sufiicient to effect coagulation of the precipitate.

'7. The method of improving the hiding power of pigments which comprises forming an aqueous slurry of the pigment containing a water soluble silicate and adding aluminum sulfate and a water soluble substance selected from the class consisting of zinc and alkaline earth metal oxides,

hydroxides and salts, and alkaline metal oxides and hydroxides, in amounts sufiicient to precipitate the silicate in insoluble form and to efiect coagulation of the precipitate.

8. The method of claim 7 in which the amount of aluminum silicate is sufiicient to substantially completely precipitate the soluble silicate as aluminum silicate.

9. The method of improving the hiding power of pigments which comprises forming an aqueous slurry of the pigment containing a soluble silicate and adding aluminum sulfate and a water soluble substance selected from the class consisting of zinc and alkaline earth metal oxides, hydroxides and salts, and alkali metal oxides and hydroxides 'n amounts sufficient to precipitate the silicate in insoluble form and to effect coagulation of the precipitate, the aluminum sulfate also being in an amount in excess of that required to supply enough acid to satisfy the hydroxyl ions attributable to the alkali metal oxide of the alkali metal silicate, and the amount of the water soluble substance being suflicient to supply enough hydroxyl ions to satisfy the acidity due to the excess aluminum sulfate.

10. The method of improving the hiding power of a pigment which comprises forming an aqueous slurry of a pigment containing a water soluble silicate, precipitating the silicate by means of aluminum sulfate in an amount in excess of that required to supply enough acid to satisfy the hydroxyl ions attributable to the alkali metal oxide of the alkali metal silicate, and mixing therewith a water soluble substance selected from the class consisting of zinc and alkaline earth metal oxides, hydroxides and salt and alkali metal oxides and hydroxides in amount suflicient to effect coagulation of the precipitate and to supply enough hydroxl ions to satisfy the acidity due to the excess aluminum sulfate.

KENNETH S. MOWLDS. 

